Swedish patent specification no. SE 8400904 discloses a method of decreasing the viscosity of black liquor after the evaporator immediately before the recovery boiler by oxidizing the liquor whereafter liquor of a higher dry-solids content can be introduced into the boiler. The oxidation reaction which is achieved by mixing warm oxidizing gas (such as air) in the liquor raises the temperature of the liquor and makes it more fluent or flowable. The method requires quite a complex apparatus for distributing the oxidizing gas evenly in the liquid. The method is primarily effected immediately before the recovery boiler where a rise in the temperature of the liquid is achieved and pumping facilitated. If the method is applied in the process before the final evaporation, the carbon dioxide and organic acids produced in the oxidizing reaction reduce the pH of the liquor which may result in condensation of the lignin and cause considerable depositions on the heat transfer surfaces.
It is a known fact that the viscosity of black liquor depends on the amount of the macro-molecular lignin in the liquor. In prior art methods the viscosity has been decreased by removing the macro-molecular fraction.
Finnish patent specification no. 66035 discloses a method of removing organic material containing a macro-molecular fraction from the effluent of a cellulose cook. According to this method the macro-molecular fraction is removed by ultra-filtering or precipitating. Precipitation is performed by oxidizing the effluent which significantly reduces the viscosity of the effluent.
The ultrafiltering method, by which the macro-molecular fraction is separated from the effluent, requires, however, sizable and expensive apparatus. A complex apparatus is required for the precipitation. Addition of chemicals in the precipitation phase affects the chemical balance of the whole system. In continuous operation, separation of the chemicals is problematic and cannot easily be controlled or changed.